In the related art, a transmission-type photoelectric encoder as shown in FIG. 15 has been used. The transmission-type photoelectric encoder as shown in FIG. 15 includes a light source 10, a collimator lens 12, a transmission-type main scale 20, and a light receiving portion 30. The collimator lens 12 forms the light emitted from the light source 10 into parallel rays. The transmission-type main scale 20 has a first grating 21 having a predetermined pitch P at a surface (lower face in the drawing) thereof. The light receiving portion 30 consists of an index grating 31 having a predetermined pitch Q and a light receiving element 32.
In the transmission-type photoelectric encoder of FIG. 15, since the index grating 31 and the light receiving element 32 are formed as individual parts, they need to be assembled to form the light receiving portion 30. Further, a plurality of light receiving elements 32A, 32B are needed for discriminating directions (A phase, B phase) and they need selection, to obtain uniform optical sensitivity and uniform temperature characteristics.
To solve such problems, as described in Japanese Publication JP-B-2610624, there is proposed a transmission-type photoelectric encoder using a light receiving element array 33 as the light receiving portion, as shown in FIG. 16 (front view) and FIG. 17 (plane view taken along line III—III of FIG. 16). That is, the light receiving element array 33 includes an index grating pattern having the predetermined pitch Q and a light receiving element 32, which are integrally formed with each other. In FIG. 17, numeral 34 designates a preamplifier and numeral 36A, 36B designate differential amplifiers.
By adopting such a light receiving element array 33, a number of advantages are achieved including realizing small-sized formation and signal stability.
Further, in a reflection-type photoelectric encoder, as described in JP-B-60-23282, a three-grating system is used, as shown in FIG. 18. That is, the reflection-type photoelectric encoder includes a first grating 51 on an index scale 50, a second grating 42 on a reflection-type main scale 40, and a third grating (index grating) 53 on the index scale 50. A bright/dark pattern obtained by the two gratings (the first grating 51 and the second grating 42) is changed by moving the gratings relative to each other. The bright/dark pattern is filtered by the third grating 53 to thereby detect a relative movement amount of the main scale 40 and the index scale 50. This reflection-type photoelectric encoder can obtain the above-described advantages by using the light receiving element array 33 (arranged to face the main scale 40 in FIG. 18) as the light receiving portion.
However, in any of the encoders described above, the grating pitch Q on the light receiving side is determined by the grating pitch P of the main scale 20, 40. Therefore, when using a main scale having a grating pitch different from the pitch P, the light receiving portion needs to be remade or replaced with a new one to correspond with the main scale having a different grating pitch. Further, the bright/dark pattern is obtained only at a distance from the surface of the grating 21, 42 of the main scale 20, 40, which distance is determined by the grating pitch P and an optical wavelength λ. Therefore, whenever a gap between the main scale and the light receiving portion changes, a signal output of the light receiving portion decreases, regardless of whether the light receiving array 33 is adopted or not.
Further, according to the encoder utilizing a three-grating system, as shown in FIG. 18, normally, the second grating 42 is formed on the main scale 40. Therefore, as shown in FIG. 19, when there is waviness at the surface of the main scale 40, a measurement error may result from a change in a reflected angle of light.
It is apparent from the foregoing description that displacement measurements are typically made by sensing the relative change in the position of a scale from a reference position relative to a photoelectric encoder readhead (which is sometimes simply referred to as a “readhead” herein) of an optical encoder. Typically, this requires sensing a periodic scale pattern so that periods of the pattern can be counted during movement, and furthermore sensing the position of the scale pattern within a particular period at the start and finish of a motion, to provide a measurement resolution that is finer than the period of the scale pattern. Providing a measurement resolution that is finer than the period of the scale pattern is often referred to as signal interpolation, measurement signal interpolation, or providing an interpolated measurement.
In addition to the previously described optical encoders, various optical encoders are known that use a readhead having a relatively simple optical arrangement that includes a lens to provide an image of the scale pattern to a photodetector arrangement in the readhead. This type of system, which images a scale pattern onto an optical reader to thereby measure the relative or absolute displacement of the scale pattern, is called an imaging-type encoder. One such system is disclosed in U.S. Pat. No. 5,539,519, to Takagi et al., which is incorporated herein by reference. The system described in the '519 patent includes an encoder plate having a periodic slit pattern. A light source illuminates the slit pattern to form a primary fringe image. A lens projects the primary fringe image by a given magnification to form a secondary enlarged fringe image that shifts along a second plane. A fixed light receiving unit receives the shifting image through a fixed periodic mask pattern (an index grating) at the second plane. However, the signals provided by an encoder according to the '519 patent are suitable only for very crude levels of measurement signal interpolation. In addition, various characteristics of the signals are not stable with respect to various potential misalignments and gap variations between the encoder readhead and scale. Thus, an encoder according to the '519 patent cannot provide robust signals suitable for significant levels of measurement signal interpolation.
Some imaging-type optical encoder readheads are known that use a relatively simple optical arrangement that further includes a telecentric aperture. A telecentric aperture provides relatively constant magnification over a desired range of object distances. However, similarly to the '519 patent, the signals provided by such known readheads also are suitable only for very crude levels of measurement signal interpolation. Thus, their resolution and accuracy are relatively crude.
A position sensing device that can overcome the foregoing problems and limitations, individually or in combination, is desirable.